Radio frequency (RF) power amplifiers may encounter antennas or other loads with impedances that may change in a number of ways. The environment around a given antenna or load may change due to environmental conditions such as the formation of ice. The antenna of a mobile device may be placed near a large metal object. Different antennas, radiating elements, or loads may be used in different applications or installations. Antenna impedance can also change when multiple antennas are used in arrays. A phased array antenna is an antenna system, having multiple radiation elements, in which the radiation pattern can be steered in particular directions by controlling the relative phases of the signal delivered to the radiation elements. The impedance of the antenna array elements in a phased array antenna changes as the array is scanned. A phased array antenna may be used for radar systems, communication systems, etc. In addition, impedance of a load may be a factor in lower frequency power amplifiers, including amplifiers operating at ultrasonic and audio frequencies.
Power amplifiers provide an amplified signal to an antenna or to other loads. According to linear circuit theory, optimum power transmission between the power amplifier (the source) and the load is achieved when the output impedance of the power amplifier (the load line impedance) is matched to the input impedance of the load (the load impedance) resulting in minimum reflection in a transmission line sense. However, as signal amplitude is increased, the voltage and current limits of the power amplifier become the dominant factor. The power amplifier will show better linearity and higher power output if the real and imaginary parts of the load impedance are consistent with the voltage and current limits of the device and the reactive (capacitive and inductive) parasitics contained in the device. The optimum condition results in the largest range of both voltage and current and is known as the load line match. The load line is the locus of points in the current-voltage plane where the device will operate for a specific given load. This locus of points can be overlaid on the current-voltage curves of the device, for example from a curve tracer. For purely resistive loads the resulting locus is a straight line, the load line. For typical devices the optimum load line runs from the highest device current at the low end of its voltage range to the lowest device current at the high end of its voltage range.
The power amplifier load line and the actual impedance of the load are usually different. A impedance matching network or device is used to transform the load impedance to the impedance of the optimum load line at the device. This will optimize power transfer between the power amplifier and load in large signal conditions. Matching to the load line impedance (through design of the power amplifier, and/or the impedance matching network) provides benefits such as increased power delivery, better efficiency, reduced localized heating (e.g., due to power loss), and increased linearity, and/or reduced inadvertent phase change with amplitude and intermodulation products.
Impedance matching may be achieved with a static impedance design (assuming the power amplifier and the load have unvarying impedances) or may be achieved with a tunable impedance design. For example, impedance matching networks may be formed of resistors, inductors, and/or capacitors (passive devices) configured to adapt the output impedance of a power amplifier into the input impedance of a load. If the resistors, inductors, and/or capacitors have static values, the impedance matching network will likewise have a static impedance conversion (e.g., at a particular frequency). If the output impedance of the power amplifier or the input impedance of the load changes, the static impedance matching network will not properly adapt the impedance between the source and load. However, there will be a one to one correspondence between the impedance at the load (e.g., antenna) and the impedance seen at the amplifying device through the transformation of the matching network. If one or more of the inductors and/or capacitors are adjustable (a tunable device), the impedance matching network can be changed to accommodate some differences in the impedance of the load to keep the impedance matched to the load line of the amplifying device.
Impedance matching networks inherently dissipate some of the power in the signal emitted from the power amplifier. Mechanically adjustable impedance matching devices can be as low in loss as fixed tuned elements. But electronically adjustable impedance matching devices are much more susceptible to the problem of loss, for example varactor diode capacitors and they may be expensive and bulky in the case of adjustable inductors or magnetic components. Electronically tunable impedance matching elements like these are generally considered impractical for most power amplifier applications.
Though static Impedance matching is efficient and low cost, and many types of loads, such as phased array antennas and antennas on mobile devices, have variable impedances that may vary significantly. For example, a phased array antenna may have an impedance variation of greater than a factor of four depending on the direction and/or type of the radiation pattern emitted. In certain cases, the loss caused by mismatched impedances may be so great that it can result in “scan blindness” in phased array antennas (where the active radiation pattern of the array may have minima at certain angles).
Hence, there is a need for efficient and adjustable load line matching systems and methods.